There was peace in Equestria for so long. Since Princess Cadence and Shining Armor banished the changelings with the power of true love there were no troubles again.
It is the fate, that brings together Firemane And her brother again, years after their family was split during the invasion of the changelings.
Old feelings come up again and at the same time the enemy of what they thought it was defeated shows up again. And it is stronger than ever!